Table of Contents

1. Six PCB Design Layout techniques

1.1 Initial schematic transfer

1.2 Handling errors in the process of schematic transfer

1.3 Update design through annotation

1.4 Renumber the device

1.5 Handle last-minute device or netlist changes

1.6 Select the positioning device by highlighting

2. Twelve PCB Design Layout Guidelines

2.1. The spacing between patches

2.2. The distance between the in-line device and the patch

2.3. For the placement of IC decoupling capacitors

2.4. Pay attention to the placement direction and distance of components on the edge of the PCB board

2.5. Pay attention to situations where adjacent pads need to be connected

2.6. If the pad falls in a normal area, heat dissipation needs to be considered

2.7. If the lead is smaller than the plug-in pad, teardrops need to be added

2.8. The lead width on both sides of the component pad should be the same

2.9. Be careful to keep the pads of unused pins and ground them

2.10. It is best not to have the via hole on the pad

2.11. Pay attention to the distance between wires or components and the edge of the board

2.12. The ambient temperature of the electrolytic capacitor must be considered away from the heat source



Six PCB Design Layout techniques
When transferring PCB schematics to layout design, we should consider 6 things. All the examples mentioned were developed using the Multisim design environment, but the same concepts apply even using different EDA tools!

1.1 Initial schematic transfer

In the process of transferring the schematic to the layout environment through the netlist file, the device information, netlist, layout information and initial trace width settings will also be transferred.

Here are some recommended steps for the layout design stage:

a. Set the grid and unit to appropriate values. In order to achieve finer layout control of components and traces, the device grid, copper-clad grid, via grid and SMD grid can be designed to be 1mil.

b. Set the blank area and vias of the circuit board outer frame to the required values. PCB manufacturers may have specific minimum or nominal recommended values ​​for blind and buried via settings.

c. Set the corresponding pad/via parameters according to the ability of the PCB manufacturer. Most PCB manufacturers can support smaller vias with a bore diameter of 10 mils and a pad diameter of 20 mils.

d. Set design rules according to requirements.

e. Set up customized shortcut keys for commonly used layers so that you can quickly switch layers (and create vias) during routing.



1.2 Handling errors in the process of schematic transfer

A common error in the schematic transfer process is non-existent or incorrect package assignment. have to be aware of is:

If there is a device in the schematic diagram that is not packaged, a warning message will pop up, indicating that the virtual component cannot be exported. In this case, no default packaging information will be passed to the layout, and the components will simply be deleted from the layout.

If the package transfer passes, but the effective package shape cannot be correctly matched, an alarm message indicating the mismatch will also be generated during the transfer process.

Correct the package assignment in the schematic or create a valid package for any device. After correction, perform the forward labeling step to update and synchronize the design information.



1.3 Update design through annotation

Annotation is the process of transferring design changes from the schematic to the layout or from the layout to the schematic. Backward labeling (from layout to schematic) and forward labeling (from schematic to layout) are the keys to keeping the design accurate.

In order to protect the work that has been completed, the current version of the schematic and layout files need to be backed up and archived before any important forward or backward annotation steps.

Do not try to make changes in the schematic and layout at the same time. Make changes to only one part of the design (either the schematic or the layout), and then perform the correct annotation steps to synchronize the design data.



1.4 Renumber the device

Device renumbering refers to a function of renumbering the components on the PCB in a specific order. The reference numbers should be sorted from top to bottom and from left to right on the PCB. This makes it easier to locate the device position on the board during assembly, testing, and troubleshooting.



1.5 Handle last-minute device or netlist changes

Last-minute PCB device or netlist changes are not desirable, but sometimes they have to be done because of device availability issues or detection of last-minute design errors. If you need to change the component or netlist, you should do it in the schematic, and then mark it to the layout tool by forward marking.

Here are some tips:

a. If a new device is added after the layout design starts (such as adding a pull-up resistor to the open-drain output), add resistors and networks to the design from the schematic diagram. After being marked in the forward direction, the resistor will be displayed outside the circuit board frame as an un-layout component, and a flying line will be displayed to indicate the connection to the network. Next, move the component to the outer frame of the circuit board and perform normal wiring.

b. Backward labeling and reference label changes can work well together, such as renumbering the rear layout.



1.6 Select the positioning device by highlighting

In the PCB layout process, one way to view specific components or traces in the schematic is to use the ‘highlight selection’ function. This function allows you to select a component or a trace (or multiple objects), and then view their position in the schematic.

This function is especially useful when matching bypass capacitors and their corresponding IC connections. Conversely, you can also locate specific components or traces in the layout when browsing the schematic.



Twelve PCB Design Layout Guidelines
2.1. The spacing between patches



The spacing between SMD components is a problem that PCB layout engineer must pay attention to during PCB Design and Layout. If the spacing is too small, it is very difficult to print solder paste and avoid soldering. The distance recommendations are as follows

Device distance requirements between patches:

a. Same kind of devices: ≥0.3mm

b. Dissimilar devices: ≥0.13*h+0.3mm (h is the maximum height difference of neighboring components)

c. The distance requirement between components that can only be manually patched: ≥1.5mm.

d. The above suggestions are for reference only and can be in accordance with the PCB manufacturing design specifications of the respective companies



2.2. The distance between the in-line device and the patch

The distance between the in-line device and the patch

As shown in the figure above, a sufficient distance should be kept between the in-line resistance device and the patch, and it is recommended to be between 1-3mm. Due to the troublesome processing, the use of straight plug-ins is rare now.



2.3. For the placement of IC decoupling capacitors

pcb layout

A decoupling capacitor must be placed near the power port of each IC, and the location should be as close as possible to the power port of the IC. When a chip has multiple power ports, a decoupling capacitor must be placed on each port.



2.4. Pay attention to the placement direction and distance of components on the edge of the PCB board

PCB board

Since the PCB is generally made of jigsaw, the devices near the edge need to meet two conditions.

a. The first is to be parallel to the cutting direction (to make the mechanical stress of the device uniform. For example, if the device is placed in the way on the left side of the figure above, when the puzzle is to be split, the force direction of the two pads of the patch is different, which may cause element components Fall off with pad).

b. The second is that components cannot be arranged within a certain distance (to prevent damage to components when the board is cut)



2.5. Pay attention to situations where adjacent pads need to be connected

5

If adjacent pads need to be connected, first confirm that the connection is made outside to prevent bridging caused by the connection, and pay attention to the width of the copper wire at this time.



2.6. If the pad falls in a normal area, heat dissipation needs to be considered

heat dissipation needs to be considered

a. If the pad falls in the pavement area, the right way should be used to connect the pad and pavement. Also, determine whether to connect 1 wire or 4 wires according to the current.

b. If the method on the left is adopted, it is more difficult to weld or repair and disassemble the components, because the temperature is fully dispersed by the copper laid, which makes the welding impossible.



2.7. If the lead is smaller than the plug-in pad, teardrops need to be added

teardrops need to be added

If the wire is smaller than the pad of the in-line device, you need to add teardrops as shown on the right side of the figure.

Adding teardrops has the following benefits:

a. Avoid the sudden decrease of the signal line width and cause reflection, which can make the connection between the trace and the component pad tend to be smooth and transitional.

b. The problem that the connection between the pad and the trace is easily broken by impact is solved.

c. Setting teardrops can also make the PCB circuit board look more beautiful.



2.8. The lead width on both sides of the component pad should be the same

lead width on both sides of the component pad should be the same

The lead width on both sides of the component pad should be the same



2.9. Be careful to keep the pads of unused pins and ground them

unused pins

Be careful to keep the pads of the unused pins and ground them properly.For example, in the picture above, two pins of a chip do not need to be used, but the physical pins of the chip exist.

If the two pins on the right side of the picture above are in a floating state, it is easy to cause interference.

If you add a pad and then ground the pad to shield it, interference can be avoided.



2.10. It is best not to have the via hole on the pad

pcb layout

Note that it is best not to punch through holes on the pads, which may easily cause solder leakage.
PCBA manufacturer


2.11. Pay attention to the distance between wires or components and the edge of the board

Note that the leads or components should not be too close to the edge of the board, especially single-sided boards. Generally, single-sided boards are mostly paper boards, which are easy to break after being stressed. If you connect or place components on the edge, it will be affected.



2.12. The ambient temperature of the electrolytic capacitor must be considered away from the heat source

pcba

Firstly, do consider whether the ambient temperature of the electrolytic capacitor meets the requirements, and secondly, keep the capacitor as far away from the heating area as possible to prevent the liquid electrolyte inside the electrolytic capacitor from drying out.
PCBA manufacturer